1. Field of the Invention
The present invention relates to a semiconductor light receiving device and a method for manufacturing the same, and particularly to a semiconductor light receiving device for repeatedly propagating an incident light in a light absorption layer and converting it to an electric signal, and a method of manufacturing the same.
2. Description of the Related Art
Conventionally, a semiconductor light receiving device formed by a semiconductor device for converting an optical signal to an electric signal is known.
FIG. 12 is a perspective view showing a constitution of a waveguide type semiconductor light receiving device as such a typical semiconductor light receiving device.
In other words, in this waveguide type semiconductor light receiving device, as shown in FIG. 12, a lower cladding layer 5 made of n-InP is formed on a substrate 6 made of n+-InP.
A light absorption layer 4 made of i-InGaAs, an upper cladding layer 3 made of p-InP, and a contact layer 2 made of p+-InGaAs are formed on the lower cladding layer 5.
A p electrode 1 is attached on an upper surface of the contact layer 2.
Further, an n electrode 7 is attached on a lower surface of the substrate 6.
Furthermore, a polyimide 8 for reducing a capacitance is formed at part of each side surface of the contact layer 2, the upper cladding layer 3, the light absorption layer 4, and the lower cladding layer 5, and a lower portion of the p electrode 1.
The light absorption layer 4 made of i-InGaAs, the upper cladding layer 3 made of p-InP, and the lower cladding layer 5 made of n-InP constitute an optical waveguide for guiding a light incident from a light incident surface of an end surface of the waveguide type semiconductor light receiving device into the inside thereof as shown in FIG. 10.
In this optical waveguide, a refractive index of the light absorption layer 4 is set to be higher than a refractive index of the upper cladding layer 3 and a refractive index of the lower cladding layer 5.
In other words, in the waveguide type semiconductor light receiving device, this light absorption layer 4 functions as a core which plays a main role for guiding the incident light.
In addition, in this waveguide type semiconductor light receiving device, the width of a mesa is on the order of 4 μm, and the length thereof is on the order of 10 μm.
In the waveguide type semiconductor light receiving device constituted in this manner, the incident light is absorbed in the light absorption layer 4 and converted to an electric signal during propagation through the optical waveguide constituted by the light absorption layer 4, the upper cladding layer 3 made of p-InP and the lower cladding layer 5 made of n-InP.
At this time, the light is absorbed according to the formula where the intensity I thereof is expressed by:I=I0 exp(−αz)  (0)
Here, I0 is a power of the incident light at the light incident end surface, α is an absorption coefficient, and z is a distance from the light incident end surface.
FIG. 13 is a diagram showing a relationship between the power I of the light guided through the optical waveguide of the waveguide type semiconductor light receiving device and the distance z.
As can be understood from the formula (0) and FIG. 13, when the light is incident into the waveguide type semiconductor light receiving device, it is exponentially attenuated.
That is, most of the light is absorbed in a short distance during propagating from the light incident end surface of the waveguide type semiconductor light receiving device through the optical waveguide, and is converted to a current.
It can be said that FIG. 13 shows a current generated because the light is absorbed, or Joule heat generated by the current, so that it can be said that, in the waveguide type semiconductor light receiving device as shown in FIG. 12, heat generation rapidly occurs in the short distance from the light incident end surface.
As a result, since, when the power of the light incident into the waveguide type semiconductor light receiving device is large, Joule heat generated in the short distance from the light incident end surface becomes remarkably larger, this waveguide type semiconductor light receiving device can be broken at worst.
Additionally, in the waveguide type semiconductor light receiving device shown in FIG. 12, illustration is omitted for simplifying the description, but actually, quaternary SCH (Separate Confinement Heterostructure) layers of InGaAsP composition having a band gap wavelength of about 1.3 μm are generally intervened above and below the light absorption layer 4 in order to make the optical waveguide multimodal.
Therefore, it is assumed that the thickness of the light absorption layer 4 is 0.6 μm and the thickness of the SCH layers in total is on the order of 2 μm, in the manufacturing steps of the waveguide type semiconductor light receiving device, the thickness of the crystal to be grown becomes as thick as 3 to 4 μm in total including other layers. Accordingly, the crystal growth itself requires a lot of time and processes such as mesa etching and the like become complicated, which is a factor for restricting a yield in manufacturing.
In order to eliminate such disadvantages, there has been proposed a semiconductor light receiving device of end surface refraction type having a structure as shown in a transverse sectional view of FIG. 14 (refer to Jpn. Pat. Appln. KOKAI Publication No. 11-195807).
Additionally, as the semiconductor light receiving device of end surface refraction type, various structures have been reported, and a typical example will be shown, here.
Specifically, as shown in FIG. 12, the lower cladding layer 5 made of n-InP, the light absorption layer 4 made of i-InGaAs, the upper cladding layer 3 made of p-InP, and the contact layer 2 made of p+-InGaAs are formed on the substrate 9 made of a semi-insulating InP (SI-IP) material.
The p electrode 1 for extracting the electric signal is attached at an upper side of this contact layer 2.
Similarly, the n type electrode 7 for extracting the electric signal is attached at the lower cladding layer 5 made of n-InP.
In such a semiconductor light receiving device of an end surface refraction type, as shown in FIG. 14, the light incident end surface 10 formed by the end surfaces of the substrate 9 and the lower cladding layer 5 is formed in an inclined manner by wet etching.
In this case, an inclination angle with respect to the upper surface (lower surface of the light absorption layer 4) of the lower cladding layer 5 of the light incident end surface 10 is about 54° from the orientation property of the crystal at the time of wet etching.
There will be described operation principles of the semiconductor light receiving device of end surface refraction type having such a structure.
After the incident light is refracted at the inclined light incident end surface 10 of the semiconductor light receiving device of end surface refraction type, it is absorbed in the light absorption layer 4 to be converted into the electric signal.
In the case of this semiconductor light receiving device of end surface refraction type, as illustrated, the incident light is incident into the entire light absorption layer 4 from below and is substantially uniformly absorbed in the entire light absorption layer 4. Accordingly, the semiconductor light receiving device of end surface refraction type is more advantageous than the waveguide type semiconductor light receiving device shown in FIG. 12 with respect to device destruction due to the generation of Joule heat.
Further, as can be understood by comparing FIG. 14 and FIG. 12, in the semiconductor light receiving device of end surface refraction type, a structure is employed in which the upper cladding layer 3 made of p-InP, the aforementioned SCH layers, and the like essential for the waveguide type semiconductor light receiving device are not required.
Therefore, as compared with the waveguide type semiconductor light receiving device, in the process of manufacturing this semiconductor light receiving device of an end surface refraction type, it is advantageous that the crystal growth or the process is easy.
In addition, in this semiconductor light receiving device of an end surface refraction type, the width of the mesa is on the order of 7 μm, and the length thereof is on the order of 14 μm.
However, even the semiconductor light receiving device of an end surface refraction type as shown in FIG. 14 has the following problems to be further solved.
Note that, when it is assumed the wavelength of the incident light is 1.55 μm, the problems which this semiconductor light receiving device of an end surface refraction type has will be described.
FIG. 15 is a diagram schematically showing a trace of the center of the light propagated through this semiconductor light receiving device of an end surface refraction type.
That is, as shown in FIG. 15, the incident light is refracted at the light incident end surface 10 of the semiconductor light receiving device of an end surface refraction type.
At this time, from Snell's law at the light incident end surface 10,n0 sin θ1=n5 sin θ2  (1)is obtained.
Here, the refractive index n0 of the air is 1 and the angle θ1 formed by the trace of the center of the incident light and the normal line of the light incident end surface 10 is 36° since the inclination angle of the light incident end surface 10 by wet etching is 54°.
Further, the refractive index n5 of the lower cladding layer 5 made of n-InP is 3.15.
The refractive index of non-doped InP is 3.17, but it is known that the refractive index of n doped InP is lower than this so that n5=3.15 lower than 3.17 is assumed, here.
Therefore, the angle θ2 formed by the light refracted at the light incident end surface 10 and the normal line of the light incident end surface 10 and the angle θ3 formed by the refracted light and the horizontal line are θ2=10.8°, and θ3=25.2°, respectively, from the formula (1) and θ3=θ1−θ2.
Next, there will be considered the incidence from the lower cladding layer 5 made of n-InP into the light absorption layer 4 (refractive index n4).
When Snell's law is employed at an interface between the lower cladding layer 5 made of n-InP and the light absorption layer 4,n5 sin θ4=n4 sin θ5  (2)is obtained.
Here, θ4=64.8° is obtained from θ4=(π/2)−θ3.
It is assumed that the refractive index n4 of the light absorption layer 4 is 3.50, θ5=54.5° is obtained from the formula (2).
This is to say, in the inside of the light absorption layer 4, the light propagates towards the upper side at the angle θ6=(π/2)−θ5=35.5° from the horizon.
Therefore, the effective absorption length for the light is 1/sin θ6=1.7 times of the thickness of the light absorption layer 4.
Additionally, it is assumed that the thickness of the light absorption layer 4 is 0.5 μm, the effective absorption length of 0.86 μm is obtained for the light.
Next, when Snell's law is employed at the interface between the light absorption layer 4 and the upper cladding layer 3 made of p-InP (refractive index n3),n4 sin θ5=n3 sin θ7  (3)is obtained.
In the p type semiconductor, a hole is a majority carrier.
Since the hole has a large mass, and the refractive index n3 of the upper cladding layer 3 made of p-InP is generally same as that of non-doped InP, when n3=3.17 is assumed, θ7=64.0° is obtained from the formula (3).
In other words, in the inside of the upper cladding layer 3 made of p-InP, the light propagates at the angle θ8=π/2−θ7=26.0° from the horizon. 
Thereafter, the light is incident into the contact layer 2 made of p+-InGaAs.
When Snell's law is employed at the interface between the upper cladding layer 3 made of p-InP and the contact layer 2 made of p+-InGaAs (refractive index n2),n3 sin θ7=n2 sin θ9  (4)is obtained.
Here, since the contact layer 2 made of p+-InGaAs has the same refractive index n2 as the light absorption layer 4 (n2=n4), θ9=θ5 is obtained.
That is, also during propagating through the contact layer 2 made of p+-InGaAs, the light is further absorbed in the distance which is 1/sin θ6=1.7 times of the thickness thereof.
Additionally, when it is assumed that the thickness of the contact layer 2 made of p+-InGaAs is 0.3 μm, the effective absorption length in the contact layer 2 made of p+-InGaAs is 0.52 μm.
However, since this contact layer 2 made of p+-InGaAs is doped at high concentration of 2×1018 cm−3 to 2×1019 cm−3 so that the internal electric field is not present, the electron to be extracted from the n electrode 7 cannot reach the n electrode 7.
Therefore, the electron and the hole are recombined, as a result, they are in vain as a current, or cannot follow the high frequency signal so that the frequency response characteristics are deteriorated.
When it is considered that the thickness of the light absorption layer 4 is in the order of 0.5 μm, the thickness of the contact layer 2 made of p+-InGaAs is so thick that it cannot be ignored, deteriorations of the efficiency by the absorption of this contact layer 2 made of p+-InGaAs and the frequency response characteristics are important problems.
Furthermore, in this semiconductor light receiving device of end surface refraction type, in order to incline the light incident end surface 10, wet etching is employed so that a lot of trouble is taken for manufacturing the light receiving device.
In addition, the inclination angle of the light incident end surface 10 formed when wet etching is performed for a sufficiently long time is determined to be substantially 54° from the orientation property of the crystal, but when the time when wet etching is actually performed is so short, this angle cannot be reached and becomes larger than 54° as shown in FIG. 16.
In this case, only part of the light is radiated on the light absorption layer 4 and thereby the efficiency is deteriorated.
On the contrary, when the time when wet etching is performed is so long, the light incident end surface 10 is retreated while this angle of 54° is maintained as shown in FIG. 17 so that the distance L between the light incident end surface 10 and the light absorption layer 4 is shorter than a designed value.
Also in this case, only part of the light is radiated on the light absorption layer 4 and thereby the efficiency is deteriorated.
In the actual manufacture of the semiconductor light receiving device, it is difficult to control this etching, which is a factor that reduces the yield in manufacturing the semiconductor light receiving device of an end surface refraction type.
There has been proposed a semiconductor light receiving device in which a relative angle of the incident light and the light incident end surface 10 is set to be 60° instead of the oblique light incident end surface 10 in the semiconductor light receiving device of end surface refraction type shown in FIG. 14 by forming the light incident end surface 10 by not wet etching but cleaving (refer to Jpn. Pat. Appln. KOKAI Publication No. 2000-243984).
In other words, as shown in FIG. 18, in this semiconductor light receiving device, since the light incident end surface 10 is formed by cleavage, the light incident end surface 10 is orthogonal to the upper surface or the lower surface of the semiconductor light receiving device so that the formation of the light incident end surface 10 is remarkably easy.
In this semiconductor light receiving device, as shown in FIG. 18, the lower cladding layer 5 made of n-InP, the light absorption layer 4 made of i-InGaAs, the upper cladding layer 3 made of p-InP and the contact layer 2 made of n+-InGaAs are formed on the upper surface of the substrate 6 made of n+-InP.
The p electrode 1 is attached on the upper surface of this contact layer 2.
The n electrode 7 is attached on the lower surface of the substrate 6.
This semiconductor light receiving device is fixed by a casing 12.
The light is made incident into the light incident end surface 10 from an optical fiber 11 supported by the casing 12.
The trace of the center of the incident light in the semiconductor light receiving device in which the light incident end surface 10 shown in FIG. 18 is formed by cleavage will be described with reference to FIG. 19.
When it is assumed that the refractive index of the substrate 6 made of n+-InP is n6, the formula derived from Snell's law in the light incident end surface 10 is:
 n0 sin θ1=n6 sin θ2  (5)
Here, according to the literature (Jpn. Pat. Appln. KOKAI Publication No. 2000-243984), when θ1=60.0° is assumed, θ2=16.0° is obtained using n6=3.15 from the formula (5).
Here, it is assumed that the refractive index n5 of the lower cladding layer 5 made of n-InP is same as the refractive index n6 of the substrate 6 made of n+-InP, that is 3.15, θ4=74.0° is obtained.
At the interface between the lower cladding layer 5 made of n-InP and the light absorption layer 4,n5 sin θ4=n4 sin θ5  (6)is established so that θ5=59.9° is obtained.
Here, the refractive index n4 of the light absorption layer 4 is assumed to be 3.5.
This is to say, in the inside of the light absorption layer 4, the light propagates towards the upper side at the angle θ6=π/2−θ5=30.1° from the horizon.
Therefore, the effective absorption length for the light is 1/sin θ6=2.0 times of the thickness of the absorption layer 4.
Additionally, it is assumed that the thickness of the light absorption layer 4 is 0.5 μm, the effective absorption length for the light is about 1 μm.
Next, when Snell's law is employed at the interface between the light absorption layer 4 and the upper cladding layer 3 made of p-InP,n4 sin θ5=n3 sin θ7  (7)is obtained.
Since the hole has a large mass, the refractive index n3 of the upper cladding layer 3 made of p-InP in which the hole is a majority carrier is same as that of non-doped InP so that, when n3=3.17 is assumed, θ7=72.8° is obtained from the formula (7).
In other words, in the inside of the upper cladding layer 3 made of p-InP, the light propagates at the angle θ8=π/2−θ7=17.2° from the horizon.
Thereafter, the light is incident into the contact layer 2 made of p+-InGaAs.
When Snell's law is employed at the interface between the upper cladding layer 3 made of p-InP (refractive index n3) and the contact layer 2 made of p+-InGaAs (refractive index n2),n3 sin θ7=n2 sin θ9  (8)is obtained.
Here, the contact layer 2 made of p+-InGaAs has the same refractive index n2 as the light absorption layer 4 (n2=n4) so that θ9=θ5 is immediately obtained.
Specifically, also during propagation through the contact layer 2 made of p+-InGaAs, the light is further absorbed in the distance which is 1/sin θ6=2.0 times the thickness thereof.
Additionally, when it is assumed that the thickness of the contact layer 2 made of p+-InGaAs is 0.3 μm, the effective absorption length in the contact layer 2 made of p+-InGaAs is 0.60 μm.
However, this contact layer 2 made of p+-InGaAs is doped at a high concentration of 2×1018 cm−3 to 2×1019 cm−3 so that the electric field is not applied.
Accordingly, as described above, the generated electron and the carrier such as the hole are recombined so that they are in vain as a current or cannot follow the high frequency optical signal, and the frequency response characteristics of this semiconductor light receiving device are deteriorated.
When it is considered that the thickness of the light absorption layer 4 is in the order of 0.5 μm, the thickness of the contact layer 2 made of p+-InGaAs is so thick that is cannot be ignored so that deteriorations of the efficiency by the absorption of the contact layer 2 made of p+-InGaAs and the frequency response characteristics are important problems.
In this manner, in both the semiconductor light receiving device shown in FIG. 14 and the semiconductor light receiving device shown in FIG. 18, the light obliquely upward passes through the light absorption layer 4, and then passes through the upper cladding layer 3 made of p-InP so that it is influenced by the absorption in the contact layer 2 made of p+-InGaAs. Therefore, there occurs the problem that the conversion efficiency of the light into the electric signal and the frequency response characteristics are bad.
Furthermore, in the manufacture of these semiconductor light receiving devices, as the contact layer 2 made of p+-InGaAs, metals such as Ni/Zn/Au and the like are generally deposited at first.
Thereafter, the temperature is raised to near 400° C., these metals are subjected to alloying (called alloying or sintering) with the contact layer 2 made of p+-InGaAs so that ohmic contact can be obtained.
At this time, the metal components described above enter the inside of the contact layer 2 made of p+-InGaAs, and thereby the crystallinity of the contact layer 2 is deteriorated and the smoothness of the interface between the p electrode 1 and the contact layer 2 made of p+-InGaAs is remarkably deteriorated.
Therefore, when the light reaches these areas, many carriers which do not contribute to the photoelectric conversion by the contact layer 2 made of p+-InGaAs occur and the scattering loss of the light is also large, and consequently the efficiency of the photoelectric conversion and the frequency response characteristics as the semiconductor light receiving device are deteriorated.
In this manner, in the conventional semiconductor light receiving devices shown in FIG. 14 and FIG. 18, the light passes through the light absorption layer 4 according to Snell's law in geometrical optics so that there are the problems that the conversion efficiency of the light into the electric signal is low and the frequency response characteristics are deteriorated.